System and method for controlling an engine

ABSTRACT

A system and method for controlling an engine coupled to an emission system is provided. The system includes an emission catalyst and a HEGO sensor disposed downstream of the catalyst. The method includes adjusting an air-fuel ratio of the engine to substantially maintain an output signal from the downstream HEGO sensor within a predetermined linear operating range.

[0001] This application is a continuation-in-part of application Ser. No. 09/991,519, filed Nov. 21, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to a system and method for controlling an internal combustion engine.

BACKGROUND OF THE INVENTION

[0003] A known engine control system is disclosed in U.S. Pat. No. 5,842,340. The known system includes an engine coupled to an emission catalyst with an UEGO sensor disposed upstream of the catalyst and a HEGO disposed downstream of the catalyst. The system attempts to maintain a predetermined level of oxygen in an emission catalyst using a UEGO output signal and a binary HEGO output signal. A drawback of the system however, is that relatively complex integration algorithms are utilized to estimate the oxygen stored in the catalyst when only using the binary states of the downstream HEGO output signal.

[0004] The inventors herein have recognized that the control algorithms for maintaining a catalyst at an optimal oxygen storage level can be greatly simplified. In particular, the inventors herein have recognized that a narrow linear operating range of the HEGO output signal corresponds to an optimal catalyst oxygen storage capacity range. Thus, by controlling an air-fuel ratio in the engine to maintain the HEGO output signal within the linear operating range, optimal performance of the catalyst is maintained. Further, because the inventive system utilizes the linear operating region of the HEGO sensor, the equations utilized to determine the oxygen storage level in the catalyst are greatly simplified.

SUMMARY OF THE INVENTION

[0005] The present invention provides a system, a method, and an article of manufacture for controlling an engine.

[0006] The method for controlling an engine coupled to an emission system in accordance with a first aspect of the present invention is provided. The emission system includes an emission catalyst and a HEGO sensor disposed downstream of at least a portion of the catalyst. The method includes adjusting an air-fuel ratio of the engine to substantially maintain an output signal from the downstream HEGO sensor within a predetermined linear operating range.

[0007] A system for controlling an engine coupled to a downstream emission catalyst in accordance with a second aspect of the present invention is provided. The system includes a HEGO sensor disposed downstream of at least a portion of the catalyst generating a first output signal. The system further includes a controller receiving the output signal and configured to adjust an air-fuel ratio of the engine to substantially maintain the output signal from the HEGO sensor within a predetermined linear operating range.

[0008] An article of manufacture in accordance with a third aspect of the present invention is provided. The article of manufacture includes a computer storage medium having a computer program encoded therein for controlling an engine coupled to an emission system. The system has an emission catalyst and a HEGO sensor disposed downstream of at least a portion of the catalyst generating an output signal. The computer storage medium includes code for adjusting an air-fuel ratio of the engine to substantially maintain an output signal from the downstream HEGO sensor within a predetermined linear operating range.

[0009] The system and method for controlling an engine represent a significant improvement over conventional systems and methods. In particular, the system utilizes greatly simplified oxygen storage algorithms for air-fuel control by maintaining a downstream HEGO sensor within the linear operating range of the HEGO sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic of a vehicle having an engine and a control system in accordance with the present invention.

[0011] FIGS. 2A-2E is a flowchart of a method for controlling an engine in accordance with the present invention.

[0012] FIGS. 3A-3G are signal schematics illustrating operation of the control system when maintaining the HEGO sensor output signal within a linear operating region.

[0013] FIGS. 4A-4F are signal schematics illustrating operation of the control system when the system is adjusting the estimated catalyst capacity range to induce the HEGO sensor output signal move toward a linear operating region.

[0014]FIG. 5 is a signal schematic of the HEGO sensor output signal illustrating the linear operating region of the sensor.

DESCRIPTION OF AN EMBODIMENT

[0015] Referring now to the drawings, like reference numerals are used to identify identical components in the various views. Referring to FIG. 1, an automotive vehicle 10 is shown. Vehicle 10 includes an internal combustion engine 12 and an engine control system 14 in accordance with the present invention.

[0016] Referring to FIG. 1, only one cylinder of a plurality of cylinders is shown for purposes of clarity. Engine 12 includes a combustion chamber 30, cylinder walls 32, a piston 34, a crankshaft 35, a spark plug 36, an intake manifold 38, an exhaust manifold 40, an intake valve 42, an exhaust valve 44, a throttle body 46, a throttle plate 48, a fuel injector 50, and an emission catalyst 52.

[0017] Combustion chamber 30 communicates with intake manifold 38 and exhaust manifold 40 via respective intake and exhaust valves 42, 44. Piston 34 is positioned within combustion chamber 30 between cylinder walls 32 and is connected to crankshaft 35. Ignition of an air-fuel mixture within combustion chamber 30 is controlled via spark plug 36 which delivers ignition spark responsive to a signal from distributorless ignition system 54.

[0018] Intake manifold 38 communicates with throttle body 46 via throttle plate 48. Throttle plate 48 is controlled by electric motor 55 which receives a signal from ETC driver 56. ETC driver 56 receives a control signal (DC) from a controller 58. Intake manifold 38 is also shown having fuel injector 50 coupled thereto for delivering fuel in proportion to the pulse width of signals (FPW) from controller 58. Fuel is delivered to fuel injector 50 by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (now shown). Although port fuel injection is shown, direct fuel injection could be utilized instead of port fuel injection.

[0019] Catalyst 52 reduces exhaust gas constituents such as nitrous oxides (NOx) and oxidizes carbon monoxide (CO) and hydrocarbons (HC).

[0020] Control system 14 is provided to control the operation of engine 12 in accordance with the present invention. Control system 14 includes distributorless ignition system 54, an electric motor 55 for controlling the throttle plate 48, an ETC driver 56, exhaust gas sensors 60, 64, a mass air flow sensor 68, a temperature sensor 70, a throttle position sensor 72, a torque sensor 74, an engine speed sensor 76, a pedal position sensor 78, an accelerator pedal 80, and controller 58.

[0021] Mass air flow sensor 68 generates a signal indicating the inducted mass air flow (AM) that is transmitted to controller 58. Sensor 68 may be coupled to the throttle body 46 or intake manifold 38.

[0022] Temperature sensor 70 generates a signal indicating the engine coolant temperature (ECT) received by controller 58. Sensor 70 may be coupled to cooling jacket 71 in cylinder wall 36.

[0023] Throttle position sensor 72 generates a signal indicating a throttle position (TP) of throttle plate 48 received by controller 58 for closed-loop control of plate 48.

[0024] Torque sensor 74 generates a signal (TQ) that may indicate one of following torque values: (i) an engine crankshaft torque, ii) a transmission torque, such as for example, a torque converter turbine torque or a transmission output shaft torque, or (iii) an axle torque.

[0025] Engine speed sensor 76 may comprise a hall effect sensor that generates a signal (N) indicating an engine speed. Sensor 76 may be coupled to crankshaft 35 and transmits signal (N) to controller 58.

[0026] Accelerator pedal 80 is shown communicating with a driver's foot 82. Pedal position sensor 78 generates a signal indicating acceleration pedal position (PP) that is transmitted to controller 58.

[0027] A universal exhaust gas oxygen (UEGO) 60 sensor, also known as a proportional oxygen sensor generates a signal whose magnitude is proportional to the oxygen level (and the air-fuel ratio) in the exhaust gases. UEGO sensor 60 is disposed upstream of catalyst 52 and generates a signal (UEGO_S) proportional to an oxygen concentration in the exhaust gases.

[0028] Heated exhaust gas oxygen sensor (HEGO) sensor 64 is disposed downstream of at least some portion of a catalyst 52. For example, HEGO sensor 64 could be disposed downstream a predetermined distance from catalyst 52. Alternately, for example, HEGO sensor 64 could be disposed at least partially within catalyst 52 at a predetermined location along an axial length of catalyst 52. HEGO sensor 64 generates a signal (HEGO_S) indicative of the oxygen concentration in exhaust gases downstream of catalyst 52. Referring to FIG. 5, the signal (HEGO_S) generated by the HEGO sensor 64 is illustrated. Between the oxygen concentrations values (LEAN_VAL) and (RICH_VAL), the HEGO sensor 64 generates output signal (HEGO_S) that has a linear output voltage to oxygen concentration relationship. The inventive control system 14 controls the air-fuel ratio in engine 12 to maintain the output signal (HEGO_S) within a relatively narrow linear operating region, as will be described in greater detail below.

[0029] The controller 58 is provided to implement the method for controlling an engine in accordance with the present invention. The controller 58 includes a microprocessor 84 communicating with various computer-readable storage media. The computer readable storage media preferably include nonvolatile and volatile storage in a read-only memory (ROM) 86 and a random-access memory (RAM) 88. The computer readable media may be implemented using any of a number of known memory devices such as PROMs, EPROMs, EEPROMs, flash memory or any other electric, magnetic, optical or combination memory device capable of storing data, some of which represent executable instructions, used by microprocessor 84 in controlling engine 12. Microprocessor 84 communicates with various sensors and actuators (discussed above) via an input/output (I/O) interface 90. Of course, the present invention could utilize more than one physical controller to provide engine/vehicle control depending upon the particular application.

[0030] Referring to FIGS. 3A-3G, the signals utilized for controlling the engine 12 will now be explained. As shown in FIG. 3A, the signal (UEGO_S) corresponds to output signal generated by the UEGO sensor 60. The value (UEGO_VAL) corresponds to a sampled voltage of the signal (UEGO_S). The signal (HEGO_S) corresponds to the output signal generated by the HEGO sensor 64. The value (HEGO_VAL) corresponds to a sampled voltage of the signal (HEGO_S).

[0031] Referring to FIGS. 3B, 3C, 3G, the catalyst state (CAT_STATE) is a scalar value indicative of the current amount of oxygen stored in catalyst 52. When the output signal (HEGO_S) is within its linear operation region, the value (CAT_STATE) is between the scalar values “−1” and “1” and optimal reduction of nitrogen oxides (NOx) and oxidation of hydrocarbons (HC) and carbon monoxide (CO) is obtained. In this case, the controller 58 sets the (INTEGRATION_ENABLE) flag equal to “1” indicating the oxygen capacity range (CAT_CAPACITY) of catalyst 52 is being updated. The value (CAT_CAPACITY) corresponds to a desired range of the stored oxygen amount in catalyst 52, with the desired oxygen range being centered about a desired scalar value “0” indicative of the optimal amount of stored oxygen in catalyst 52.

[0032] When the

[0033] the output signal (HEGO_S) is not within its linear operation region, controller 58 sets the (INTEGRATION_ENABLE) flag equal to “0” indicating the calculation of the oxygen capacity range (CAT_CAPACITY) is being discontinued.

[0034] Referring to FIG. 3D, the air-fuel control signal (LAMBSE) is illustrated. The controller 58 adjusts the signal (LAMBSE) to maintain the signal (HEGO_S) within its linear operating region. In particular, the controller 58 adjusts the value (CAT_CAPACITY) in order for the signal (CAT_STATE) to correlate with the HEGO output signal (HEGO_S). In other words, the value (CAT_STATE) should indicate a relatively large amount of stored oxygen (e.g., CAT_STATE=1) when the value (HEGO_VAL) indicates a lean air-fuel ratio. Further, the value (CAT_STATE) should indicate a relatively small amount of stored oxygen (e.g., CAT_STATE=−1) in catalyst 52 when the value (HEGO_VAL) indicates a rich air-fuel ratio.

[0035] Referring to FIGS. 3E, 3F the signal (CAT_STATE_RMS) corresponds to the root mean square value of the signal (CAT_STATE). Further, the value (HEGO_VAL_RMS) corresponds to the root mean square of the value (HEGO_VAL). The ratio of the signals (CAT_STATE_RMS) and (HEGO_VAL_RMS) are utilized by controller 58 for providing minor adjustments to the catalyst capacity range (CAT_CAPACITY). In particular, when the ratio of (HEGO_VAL_RMS)/(CAT_STATE_RMS) is greater than a predetermined threshold, the ratio indicates that the variations in the estimated catalyst state (CAT_STATE) is smaller than expected when compared to the measured variations in the HEGO output signal (HEGO_S). In other words, the ratio suggests that the estimated catalyst capacity range (CAT_CAPACITY) is larger than the actual catalyst capacity range, the actual catalyst capacity range corresponding to the linear range of HEGO sensor 64. In this case, the controller 58 decreases (CAT_CAPACITY), as will be explained in greater detail below.

[0036] Alternately, when the ratio of (HEGO_VAL_RMS)/(CAT_STATE_RMS) is less than the predetermined threshold, the ratio indicates that the variations in the estimated catalyst state (CAT_STATE) is larger than expected when compared to the measured variations in the HEGO output signal (HEGO_S). In other words, the ratio suggests that the estimated catalyst capacity range (CAT_CAPACITY) is smaller than the actual catalyst capacity range. As a result, controller 58 increases (CAT_CAPACITY), as will be explained in greater detail below.

[0037] Referring to FIG. 4E, the signals (RICH_CNT) and (LEAN_CNT) correspond to time weighted counts utilized by controller 58 for aggressively decreasing the value (CAT_CAPACITY) when the catalyst 52 is substantially depleted of oxygen or substantially saturated with oxygen, respectively.

[0038] Referring to FIGS. 4A-4F, the operation of the control system 14 will be explained during non-stabilized operating conditions. As shown, between T=221 seconds and T=224 seconds, the HEGO output signal (HEGO_S) is greater than 0.7 volts (i.e., RICH_VAL) which is outside the linear operating range of HEGO sensor 64. Accordingly, controller 58 sets the value (CAT_STATE) equal to “−1” indicating the catalyst 52 is saturated rich. Further, controller 58 disables the (INTEGRATION_ENABLE) flag so that the value of (CAT_STATE) will be maintained at the value “−1”. Further, controller 58 ramps the air-fuel control signal (LAMBSE) toward a leaner air-fuel value in order to return the system 14 back to the desired state (i.e., CAT_STATE=0).

[0039] Between T=224-224.5 seconds, the HEGO output signal (HEGO_S) decreases into the desired linear operating range (e.g., 0.2-0.7 volts) and controller 58 updates the value (CAT STATE) by setting the (INTEGRATION_ENABLE) flag equal to “1”. Further, controller 58 adjusts the air-fuel control signal (LAMBSE) towards a richer air-fuel ratio as the estimated state (CAT_STATE) moves toward the desired state (i.e., CAT_STATE=0).

[0040] At T=224.5 seconds, the output signal (HEGO_S) falls below 0.2 volts indicating the catalyst 52 is saturated lean. As a result, controller 58 sets the value (CAT_STATE) equal to “1” indicating the catalyst 52 is saturated lean. Further, controller 58 disables the (INTEGRATION_ENABLE) flag so that the value of (CAT_STATE) will be maintained at the value “1”. Further, controller 58 ramps the air-fuel control signal (LAMBSE) aggressively toward a richer air-fuel ratio in order to return the system 14 back to the desired state (i.e., CAT_STATE=0).

[0041] At T=225.5 seconds, the output signal (HEGO_S) again moves into the linear operating region. As a result, controller 58 updates the value (CAT_STATE) by setting the (INTEGRATION_ENABLE) flag equal to “1”. Further, controller 58 adjusts the air-fuel control signal (LAMBSE) towards a leaner air-fuel ratio as the estimated state (CAT_STATE) moves toward the desired state (i.e., CAT_STATE=0). However, at T=227 seconds, the output signal (HEGO_S) again moves outside the linear operation region.

[0042] Referring to FIGS. 3A-3C, the operation of control system 14 will be explained under stabilized operating conditions. As shown, the HEGO output signal (HEGO_S) is maintained within the linear operating range (e.g., 0.2-0.7 volts). As a result, the value (CAT_STATE) indicative of the amount of stored oxygen in catalyst 52, is maintained between the desired scalar limits of “1” and “−1”. Further, because the HEGO output signal (HEGO_S) is maintained with the desired linear operating range, optimal catalyst performance is obtained from catalyst 52.

[0043] Referring to FIGS. 2A-2E, a flowchart of the inventive method for controlling an engine will now be discussed. The method may be implemented using software stored in memory 86 of controller 58.

[0044] Initially at step 92, the signal (UEGO_S) from UEGO sensor 60 is sampled by controller 58 and the corresponding value (UEGO_VAL) is stored in memory 88.

[0045] Next at step 94, the signal (HEGO_S) from HEGO sensor 64 is sampled by controller 58 and the corresponding value (HEGO_VAL) is stored in memory 88.

[0046] Next at step 96, the long-term air-fuel correction value (UEGO_CORR), also known as a stoichiometric reference value, is determined. Referring to FIG. 2B, steps 98-102 are utilized to calculate (UEGO_CORR). At step 98, the value (HEGO_ERROR) which is indicative of the difference of the signal (HEGO) from a desired value, is calculated using the following equation:

HEGO_ERROR=HEGO_TARGET_VOLTAGE-HEGO_(—) VAL

[0047] where HEGO_TARGET_VOLTAGE corresponds to a stoichiometric oxygen concentration, such as 0.6 volts for example

[0048] Next at step 100, an (INTEGRAL_CORRECTION_VALUE) is calculated using the following equation:

INTEGRAL_CORRECTION_VALUE=G1* HEGO_ERROR+INTEGRAL_CORRECTION_VALUE_(I-1)

[0049] where G1 corresponds to a gain value that is determined empirically.

[0050] Next at step 102, the correction value (UEGO_CORR) is calculated using the following equation:

UEGO_(—) CORR=1+INTEGRAL_CORRECTION_VALUE

[0051] Referring again to FIG. 2A, after step 96, the method advances to step 104 which determines whether the value (HEGO_VAL) is less than the value (LEAN_VAL). The value (LEAN_VAL) corresponds to the lower lean limit of the linear operating region of output signal (HEGO_S) of the HEGO sensor 64. If the value of step 104 is True, indicating a lean-air fuel ratio, the step 106 sets the catalyst state variable (CAT_STATE) equal to the value “1”. Next, the step 108 sets the flag (INTEGRATION_ENABLE) equal to the value “0” and the method advances to step 124.

[0052] Referring again to step 104, if the value of step 104 is False, the method advances to step 110 which determines whether the value (HEGO_VAL) is greater than the value (RICH_VAL). The value (RICH_VAL) corresponds to the upper rich limit of the linear operating region of the output signal (HEGO_S) of the HEGO sensor 64. If the value of step 110 is True, the step 112 sets the value (CAT_STATE) equal to the value “−1”. Next, the step 114 sets the flag (INTEGRATION_ENABLE) equal to the value “0” and the method advances to step 124.

[0053] Referring again to step 110, if the value of step 110 is False, the step 116 sets the flag (INTEGRATION_ENABLE) equal to the value “1” and the step 118 calculates the value (CAT_STATE).

[0054] Referring to FIG. 2C, the steps 120, 122 used to implement the step 118 will be explained. At step 120, the rate of change of the catalyst state (CAT_STATE_DOT) is calculated using the following equation:

CAT_STATE_DOT=[MAF*(UEGO_(—) VAL−UEGO_(—) CORR)]/CAT_CAPACITY_(I-1)

[0055] where:

[0056] MAF corresponds to the mass flow rate into the intake manifold 38;

[0057] UEGO_VAL corresponds to voltage of the signal (UEGO_S);

[0058] CAT_CAPACITY_(I-1) corresponds to the oxygen storage capacity range of the catalyst 52.

[0059] Next at step 122, the catalyst state value (CAT_STATE) is calculated using the following integration equation:

CAT_STATE=CAT_STATE_DOT*ΔT+CAT_STATE_(I-1)

[0060] where ΔT corresponds to the elapsed time since the last updated calculation of the value (CAT_STATE).

[0061] Referring again to FIG. 2A, after any of steps 108, 114, 118, the method advances to step 124 which calculates the catalyst oxygen capacity range (CAT_CAPACITY). Referring to FIG. 2D, the steps utilized to implement step 124 will now be discussed.

[0062] At step 126, a determination as to whether the value (HEGO_VAL) is greater than the value (RICH_VAL). If the value of step 126 is True, the step 128 calculates the value (RICH_CNT) using the following equation:

RICH_(—) CNT=RICH_(—) CNT _(i-1) +C2

[0063] RICH_CNT corresponds to a time weighted count for aggressively decreasing the value (CAT_CAPACITY) when the catalyst 52 is substantially depleted of oxygen for a relatively large period of time;

[0064] C2 corresponds a constant value, such as 4 for example. Alternately, if the value of step 126 is False, the step 130 calculates the value (RICH_CNT) using the following equation:

RICH_(—) CNT=MAX [(RICH_(—) CNT−1),)0]

[0065] Thus, the value (RICH_CNT) is equal to the greater of the value RICH_CNT−1 or the value “0”.

[0066] After either of steps 128, 130, the method advances to step 132 which determines whether the value (HEGO_VAL) is less than the value (LEAN_VAL). If the value of step 132 is True, the step 134 calculates the value (LEAN_CNT) using the following equation:

LEAN_(—) CNT=LEAN_(—) CNT _(i-1) +C2

[0067] LEAN_CNT corresponds to a time weighted count for aggressively decreasing the value (CAT_CAPACITY) when the catalyst 52 is substantially saturated with oxygen for a relatively large period of time;

[0068] C2 corresponds to a constant value, such as 4 for example. Alternately, if the value of step 132 is False, the step 136 calculates the value (LEAN_CNT) using the following equation:

LEAN_(—) CNT=MAX [(LEAN_(—) CNT _(i-1)−1),)0]

[0069] Thus, the value (LEAN_CNT) is equal to the greater of the value (LEAN_CNT−1) or the value “0”.

[0070] After either of steps 134, 136, the method advances to step 138 which makes a determination of whether the value (RICH_CNT) is greater than the value (THRESHOLD1) and whether the value (LEAN_CNT) is greater than the value (THRESHOLD1). If the value of step 138 is True, the step 140 decreases the value (CAT_CAPACITY) using the following equation:

CAT_CAPACITY=CAT_CAPACITY_(i-1) −C3

[0071] After step 140, the method advances to step 148 which will be discussed in further detail below.

[0072] Referring again to step 138, if the value of step 138 is False, the step 142 mathematically filters the value (HEGO_VAL) to remove an average DC value, using the following equation:

MEAN_HEGO=((1−FC1)*MEAN_HEGO_(I-1))*(FC1*HEGO_(—) VAL))

[0073] where FC1 is a filter constant that is empirically determined.

[0074] Next at step 144, the root-mean square of the value (HEGO_VAL) is calculated using the following equation: ${{HEGO\_ VAL}{\_ RMS}} = {{\left( {1 - {FC2}} \right)*{HEGO\_ VAL}{\_ RMS}_{i - 1}} + {{FC2}*\sqrt{\left( {{HEGO\_ VAL} - {MEAN\_ HEGO}} \right)^{2}}}}$

[0075] Next at step 146, the root-mean square of the value (CAT_STATE) is calculated using the following equation: ${{CAT\_ STATE}{\_ RMS}} = {{\left( {1 - {FC2}} \right)*{CAT\_ STATE}{\_ RMS}_{i - 1}} + {{FC2}*\sqrt{({CAT\_ STATE})^{2}}}}$

[0076] It should be noted that other equations for measuring the variation of the values (HEGO_VAL) and (CAT_STATE) could have been used, instead of the two foregoing equations. For example, equations utilizing peak-to-peak values, standard deviation of the values, or the average difference between measured and averaged values could be used.

[0077] After either of steps 140, 146, the step 148 determines whether the ratio (HEGO_VAL_RMS/CAT_STATE_RMS) is greater than a value (THRESHOLD2). The value (THRESHOLD2) is determined empirically and may have a value of 0.2 for example. If the value of step 148 is True, the step 150 decreases the value (CAT_CAPACITY) using the following equation:

CAT_CAPACITY=CAT_CAPACITY_(i-1) −C4

[0078] where the constant C4 may have a value of 0.1 for example. Alternately, if the value of step 148 is False, the step 152 increases the value (CAT_CAPACITY) using the following equation:

CAT_CAPACITY=CAT_CAPACITY_(i-1) +C4

[0079] After either of steps 150, 152, the method advances to step 154 of FIG. 2A. At step 154, the commanded air-fuel value (LAMBSE) is calculated using steps 156-162 of FIG. 2E. At step 156, the value (CAT_STATE_ERROR) is calculated using the following equation:

CAT_STATE_ERROR=CAT_STATE−TARGET_STATE

[0080] where the TARGET_STATE corresponds to an oxygen concentration indicative of stoichiometry.

[0081] Next at step 158, an integration value (INTEG_VAL) is calculated using the following equation:

INTEG _(—) VAL=(KI*ΔT*CAT_STATE_ERROR)+INTEG _(—) VAL _(I-1)

[0082] where KI is a predetermined gain that is empirically determined; ΔT corresponds to the elapsed time since the last updated calculation of the value (INTEG_VAL).

[0083] Next at step 160, the value (COMMANDED_CAT_STATE) is calculated using the following equation:

COMMANDED_(—) CAT_STATE=(KP*CAT_STATE_ERROR)+INTEG _(—) VAL

[0084] where KP is a predetermined gain that is empirically determined. The foregoing equation comprises a simple proportional/integral equation for calculating the (COMMANDED_CAT_STATE). However, other control equations known to those skilled in the art could be utilized.

[0085] Next at step 162, the value (LAMBSE) is calculated using the following equation: ${LAMBSE} = \frac{\left( {1 + {{COMMANDED\_ CAT}{\_ STATE}}} \right)*{CAT\_ CAPACITY}}{MAF}$

[0086] Referring again to FIG. 2A, after step 154, the method advances back to step 92.

[0087] The above-described system and method for controlling an engine represent a significant improvement over conventional systems and methods. In particular, the system utilizes greatly simplified oxygen storage algorithms for air-fuel ratio in an engine by maintaining a downstream HEGO sensor within the linear operating region of the HEGO sensor. 

We claim:
 1. A method for controlling an engine coupled to an emission system, the system having an emission catalyst and a HEGO sensor disposed downstream of at least a portion of the catalyst, the method comprising: adjusting an air-fuel ratio of said engine to substantially maintain an output signal from said downstream HEGO sensor within a predetermined linear operating range.
 2. The method of claim 1 wherein said system includes an UEGO sensor disposed upstream of said catalyst, said adjusting step including: determining a catalyst state indicative of an amount of oxygen stored in said emission catalyst based on an output signal from said UEGO sensor; determining a catalyst oxygen capacity range based on said HEGO sensor; and, generating a commanded air-fuel signal based on said catalyst state and said catalyst oxygen capacity range.
 3. A system for controlling an engine coupled to a downstream emission catalyst, comprising: a HEGO sensor disposed downstream of at least a portion of the catalyst generating a first output signal; and, a controller receiving said output signal, said controller configured to adjust an air-fuel ratio of the engine to substantially maintain said output signal from said HEGO sensor within a predetermined linear operating range.
 4. The system of claim 3 further including an UEGO sensor disposed upstream of said catalyst generating a second output signal, said controller being further configured to determine a catalyst state indicative of an amount of oxygen stored in said emission catalyst based on said second output signal, said controller being further configured to determine a catalyst oxygen capacity range based on said first output signal, said controller configured to further generate a commanded air-fuel signal based on said catalyst state and said catalyst oxygen capacity range.
 5. An article of manufacture, comprising: a computer storage medium having a computer program encoded therein for controlling an engine coupled to an emission system, the system having an emission catalyst an a HEGO sensor disposed downstream of at least a portion of the catalyst generating an output signal, the computer storage medium comprising: code for adjusting an air-fuel ratio of said engine to substantially maintain an output signal from said downstream HEGO sensor within a predetermined linear operating range. 